My DD2 had something identical a few weeks ago. There was no accident or anything that we knew of but when trying to get her out at the shops she complained and wanted to be picked up. She then limped everytime she got out of the car, so we thought maybe pins & needles. After a weekend of her playing like normal but limping after sleeps etc, we took her to the Dr who said to give her regular doses of nurofen and return if she was still limping at the end of the week. About 2 days later she was no longer limping and hasn't since. I still have no idea what it was all about!